Hi guys, I have an update regarding my mosfet troubles. To keep the story short……….:

Last weekend I got the replacement mosfet, I installed it, SUCCESS!!! According to its markings, it was a couple of years older than the other one, and it needed abt. 0.5 volt higher bias voltage, but this is fine once adjusted.

Then I started testing again, this time with a more mature swr protection system, this system tripped immediately. Hmmmmm. What could be wrong???

OK, fast forward, back to the time of breakdown event: It was a late weekend evening. The operator may have been a bit tired, eager to produce some results. There may have been a couple of beer cans in the way. 🙂 And the shack do not look good in daylight… this does not help at midnight, but it may appear better, visually.

I am now, pretty sure that the cause of the mosfet breakdown was a missing load, not spikes or over drive from the driver. Looking for the cause for the high swr situation mentions above, I found that the TX relay (narda 023 series) had a short due to internal welding!!!! Also, the coax connected to the transmitter port had a short. The RG-142 Teflon center insulator had developed a puncture, shorting it out!!!!! There have been some serious voltage on the loose here.

Well. I repaired the relay and exchanged the damaged coax. Up and running again, but not for long. A piece of advice; knowing about their internals, relays like the narda 023s, do not abuse them. They are high cost, wide band, low insertion loss, high power relays, but keep them below the rated power limit. I think these are rated to at abt. 1Kw @ 100 MHZ @ low vswrs conditions. Well, I tested it for key down, 2Kw at 3 minutes and it would not operate any more. Wye?? Because the internal plastic bits has melted and permanently deformed.

Looking for a better replacement I discovered that an electromechanically solution does not in practice exist at these power levels. Not what my wallet is going to accept any way.

Then I come to think of SM5BSZ Leifs page, about High Power PIN Diode Switch. Well, I do not have suitable PIN diodes, but I have many different relays. Let’s try that instead. …….And it works. Only trouble is that my Q is to high. RF-current melt the solder and things broke down……again. But this can easily be rectified.

This is a drawing of the voltage & current sensing board. If you look further down on my site, this is the board on top of the rf-deck with red wires going through blue hall-effects elements. There is some more stuff on this board, and this is the VDD FET switches, but these are not in detail in the drawing.

Last night, during light burn-in tests, the amplifier suddenly dropped gain.
No alarms, no nothing, but one of the MOSFETS was drawing a lot less current compared to the other, it was also operating in class C. It turns out that for some reason the gate2 on the failing MOSFET has broken down. DAMN!!!!!!!!

Digging in to the RF-deck

Failed MOSFET

Gate2 to source breakdown. 70 ohm dc-path between source and gate2 do not make any sense.

During last weekend I was playing with a temporary setup of an old Cushcraft 13B2 145MHz yagi that I had lying around. The aim was to try and decode some EME traffic with a simple station setup, but the old 13B2 did not yield any result. Then I decided to build a more efficient antenna by myself.
After considering many design options I decided to build a 12-El.-Yagi EF2012B modified for 50 Ohm-V and optimized by DK7ZB. Here is a preliminary progress report:

Antenna schematics.

Antenna ready for the first VNA test on a stub, abt. 1 meter above ground.

VNA results not so good. I suspect that the elevation rotor is to close to the boom. I have to make a new rotor arm to get the rotor out of the way. …………….to be continued

LATEST UPDATE:

Been working on the matching problem all evening and it has nothing to do with the proximity to the rotor.It’s all about fine tuning this part, the radiator. Trimming the V-dipole length and bending angle did the trick.

Now it looks very close to the EZNEC simulation.I think there may be potential for better return-loss by some more fine tuning, but it is not going to happen tonight!

MORE PIX.

From the rear.

It is not easy to find room for all this airborne metal in a small back yard. 🙂

I’m trowing in a couple of pictures of the FOX DELTA ST2 Antenna Tracking Interface that I have build.
Unit works great, but on the bench for the moment. Antennas ain’t finished yet. 🙁
I’m in the proccess of building a 144Mhz 4×7-Element-Yagi array for EME. ETA not known.